A History of Physics in its Elementary Branches

A History of Physics in its Elementary Branches including the Evolution of Physical Laboratories by Florian Cajori was first published in 1899. It was written with the hope to improve science teaching, which Cajori felt at the time, was not sutticiently historical.

Contents

This history is intended mainly for the use of students and teachers of physics. The writer is convinced that some attention to the history of a science helps to make it attractive, and that the general view of the development of the human intellect, obtained by reading the history of science, is in itself stimulating and liberalizing.

In the announcement of Ostwald'sKlassiker der Exakten Wissenschaften [Classics of the Exact Sciences] is the following significant statement: "While, by the present methods of teaching, a knowledge of science in its present state of advancement is imparted very successfully, eminent and far-sighted men have repeatedly been obliged to point out a defect which too often attaches to the present scientific education of our youth. It is the absence of the historical sense and the want of knowledge of the great researches upon which the edifice of science rests." It is hoped that the survey of the progress of physics here presented may assist in remedying this defect so clearly pointed out by Professor Ostwald.

In mathematics, metaphysics, literature, and art the Greeks displayed wonderful creative genius, but in natural science they achieved comparatively little. ...it is true that, as a rule, they were ignorant of the art of experimentation, and that many of their physical speculations were vague, trifling, and worthless.

As compared with the vast amount of theoretical deduction about nature, the number of experiments known to have been performed by the Greeks is surprisingly small. Little or no attempt was made to verify speculation by experimental evidence.

Mechanical subjects are treated in the writings of Aristotle. The great peripatetic had grasped the notion of the parallelogram of forces for the special case of the rectangle. He attempted the theory of the lever, stating that a force at a greater distance from the fulcrum moves a weight more easily because it describes a greater circle. He resolved the motion of a weight at the end of the lever into tangential and normal components. The tangential motion he calls according to nature; the normal motion contrary to nature. ...the expression contrary to nature applied to a natural phenomenon is inappropriate and confusing.

Aristotle's views of falling bodies are very far from the truth. ...He says "That body is heavier than another which, in an equal bulk, moves downward quicker." In another place he teaches that bodies fall quicker in exact proportion to their weight. No statement could be further from the truth. ...If it had only occurred to him, while walking up and down the paths near his school in Athens, to pick up two stones of unequal weight and drop them together, he could easily have seen that the one of, say, ten times the weight, did not descend ten times faster.

Immeasurably superior to Aristotle as a student of mechanics is Archimedes. He is the true originator of mechanics as a science. To him we owe the theory of the centre of gravity (centroid) and of the lever. In his Equiponderance of Planes [On the Equilibrium of Planes] he starts with the axiom that equal weights acting at equal distances on opposite sides of a pivot are in equilibrium, and then endeavours to establish the principle that "in the lever unequal weights are in equilibrium only when they are inversely proportional to the arms from which they are suspended."

In his Floating Bodies Archimedes established the important principle, known by his name, that the loss of weight of a body submerged in water is equal to the weight of the water displaced, and that a floating body displaces its own weight of water. Since the days of Archimedes able minds have drawn erroneous conclusions on liquid pressure. The expression "hydrostatic paradox" indicates the slippery nature of the subject. All the more must we admire the clearness of conception and almost perfect logical rigour which characterize the investigations of Archimedes.

It is reported that he astonished the court of Hieron by moving heavy ships by aid of a collection of pulleys. To him is ascribed the invention of war engines, and the endless screw ("screw of Archimedes") which was used to drain the holds of ships.

The Greeks invented the hydrometer, probably in the fourth century AD. ...It was first used in medicine to determine the quality of drinking water, hard water being at that time considered unwholesome. According to Desaguliers it was used for this purpose as late as the eighteenth century.

Optics is one of the oldest branches of physics. A converging lens of rock crystal is said to have been found in the ruins of Nineveh.

In Greece burning glasses seem to have been manufactured at an early date. Aristophanes in the comedy of The Clouds... introduces a conversation about "fine transparent stone (glass) with which fires are kindled," and by which, standing in the sun, one can, "though at a distance, melt all the writing" traced on a surface of wax.

The Platonic school taught the rectilinear propagation of light and the equality of the angle of incidence to that of reflection.

The astronomer Claudius Ptolemy... measured angles of incidence and of refraction and arranged them in tables.

Metallic mirrors seem to have been manufactured in remote antiquity. Looking glasses are referred to in Exodus 38:8, and in Job 37:18; they have been found in graves of Egyptian mummies.

Spherical and parabolic mirrors were known to the Greeks. To Euclid... is attributed a work on Catoptrics, dealing with phenomena of reflection. In it is found the earliest reference to the focus of a spherical mirror. In Theorem 30 it is stated that concave mirrors turned toward the sun will cause ignition. In the "fragmentum Bobiense," a document written, perhaps, by Anthemius of Tralles, the focal property of parabolic reflectors is demonstrated. Several Greek authors appear to have written on concave mirrors.

The Greeks elaborated several theories of vision. According to the Pythagoreans, Democritus, and others vision is caused by the projection of particles from the object seen, into the pupil of the eye. On the other hand Empedocles, the Platonists, and Euclid held the strange doctrine of ocular beams, according to which the eye itself sends out something which causes sight as soon as it meets something else emanated by the object.

Amber—a mineralized yellowish resin—was used in antiquity for decoration. In common with the bright shining silver-gold alloys, and gold itself, it was called "electron"; hence the word "electricity."

Theophrastus, in his treatise On Gems mentions another mineral which becomes electrified by friction. We know now that all bodies can be thus electrified.

The polarity of magnets and the phenomenon of repulsion which may exist between electric charges or magnetic poles were unknown to Greek antiquity.

It is in Athens that we find the oldest contrivance for observing the direction of the wind. There, in its essential parts standing to this day, is the "tower of the winds," built about 100 B.C. Upon an octagon of marble was a roof, the highest part of which carried a weather-vane in form of a triton.

Among the Greeks meteorology can hardly be said to have risen to the dignity of a science.

Theophrastus of Eresus... wrote a book On Winds and on Weather Signs, but like most other Greek philosophers, he was hardly the man to adopt patient and exact observation in place of dogmatic assertion and the teaching of authority.

Aristotle makes a good observation on the formation of dew; viz. dew is formed only on clear and quiet nights.

Aratus of Soli... wrote a book of Prognostics, giving predictions of the weather from observation of astronomical phenomena, and various accounts of the effect of weather on animals.